This invention relates to a thermosensitive recording medium and, more specifically, to an improved thermosensitive recording medium having a support and a thermosensitive coloring layer formed thereon and capable of forming a color image when heated imagewise.
Generally, a thermosensitive recording medium prepared by forming on a support, such as a sheet of paper or a film, a thermosensitive coloring layer which includes as the main component a thermosensitive coloring composition is capable of producing a colored image by heating with a thermal head, thermal pen, laser beam or stroboscopic lamp. This type of recording material is advantageous over other conventional recording materials because the recording can be speedily achieved by a simple process with a relatively compact device. Noise development and environmental problems can be minimized, and the manufacturing cost of the recording material is low. Owing to the above-mentioned advantages, the thermosensitive recording medium is utilized in a wide number of fields such as facsimile machines, electronic computer terminals, recorders and automatic vending machines for labels and tickets.
The thermosensitive coloring composition for use in such a thermosensitive recording medium includes a coloring agent and a color developer capable of inducing color formation in the coloring agent upon application of heat thereto. Specifically, a colorless to light colored leuco dye with a lactone or lactam, or a spiroyran ring is employed as the coloring agent. An organic acid or phenolic material is conventionally used as the color developer. The recording medium employing the combination of the above-mentioned coloring agent and color developer is capable of producing images with clear tone, while maintaining a high degree of whiteness of the background. In addition, the weather-resistance of the obtained images is superior.
However, the thermosensitive recording medium has the shortcomings that the recording images easily fade when brought into contact with water or plasticizers such as dioctyl adipate (DOA) and dioctyl phthalate (DOP), and that the background of the recording medium is easily colored when brought into contact with oil, or solvents such as ethanol and ethyl acetate. The reliability of the images recorded on the thermosensitive recording medium is not satisfactory.
To solve the aforementioned reliability problem, the provision of a protective layer on the thermosensitive coloring layer has been proposed in, for example, Japanese Published Unexamined Patent Applications Nos. 54-128347 and 53-3594. In addition, it is disclosed in Japanese Published Unexamined Patent Applications Nos. 56-126193 and 56-13993 that the protective layers including water-soluble resins, for example, vinyl alcohol or modified materials thereof, and starch or modified materials thereof are effective in improving resistance to oil and plasticizer of the obtained thermosensitive recording medium.
However, the water resistance of such a protective layer including the water-soluble resin is poor so that the protective layer is easily peeled off when brought into contact with water. To improve the water resistance, the protective layer is often prepared by curing with a crosslinking agent as is outlined in Japanese Published Unexamined Patent Applications Nos. 57-188392. Various combinations of the water-soluble resin and the crosslinking agent are conventionally proposed.
For example, the combination of polyvinyl alcohol and an epoxy compound with two or more epoxy groups or an ethylene-imine compound with two or more ethylene-imine groups is proposed in Japanese Published Unexamined Patent Application No. 60-68990. The combination of casein and a compound having a methylol group has been proposed in Japanese Published Unexamined Patent Application No. 60-193693. The combination of a copolymer of acrylamide-crylonitrile--a carboxyl group-containing vinyl compound and an epoxy resin is proposed in Japanese Published Unexamined Patent Application No. 61-37467. The combination of polyvinyl alcohol and a 2,4-dihydroxy-1,4-dioxane derivative is proposed. in Japanese Published Unexamined Patent 10 Application No. 62-44494. Japanese Published Unexamined Patent Application No. 62-270382 discloses the use of bis(4-glycidyloxyphenyl)sulfone as a crosslinking agent. Japanese Published Unexamined Patent Applications Nos. 5-318926 and 7-164742 suggest the use of a core-shell resin for the formation of a protecting layer. The known techniques, however, are not fully satisfactory. In particular, with the known protecting layer, it is not possible to simultaneously satisfy the demands for good resistance to water, plasticizers and heat, for maintenance of the sensitivity of the thermosensitive coloring layer and for prevention of deposition of scales on a thermal head.